Laboratory Studies of Vibrational Excitation in O2(a1Δg, v) Involving

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Laboratory Studies of Vibrational Excitation in O2(a1Δg, v) Involving O2, N2, and CO2 Published as part of The Journal of Physical Chemistry virtual special issue “William M. Jackson Festschrift”. Tom G. Slanger,* Eunsook S. Hwang,†,‡ Nate C.-M. Bartlett,† and Konstantinos S. Kalogerakis*

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Center for Geospace Studies, SRI International, Menlo Park, California, United States ABSTRACT: Collisional removal of electronic energy from O2 in the low-lying a1Δg state is typically an extremely slow process for the v = 0 level. In this study, we report results on the deactivation of O2(a1Δg, v = 1−3) in collisions with O2 and CO2. Ozone photodissociation in the 200−310 nm Hartley band is the source of O2(a, v), and resonance-enhanced multiphoton ionization is used to probe the vibrational-level populations. Deactivation of the a(v = 1−3) levels in collisions with O2 at 300 K is fast, with rate coefficients of (5.6 ± 1.1) × 10−11, (3.6 ± 0.4) × 10−11, and (1.9 ± 0.4) × 10−11 cm3 s−1 (2σ) for v = 1, 2, and 3, respectively. The relaxation process appears to involve a near-resonant electronic energy transfer pathway analogous to that observed in vibrationally excited O2(b1Σ+g ). With CO2 collider gas, the removal rate coefficient at 300 K is (1.8 ± 0.4) × 10−14 and (4.4 ± 0.6) × 10−14 cm3 s−1 (2σ) for v = 1 and 2, respectively. Despite the small mole fraction of O2 in the atmospheres of Mars and Venus, O2 is at least as important as CO2 in the final stages of collisional relaxation within the O2 vibrational-level manifold.

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INTRODUCTION The a1Δg state of O2 is perhaps the most extensively studied of all metastable states.1 O2(a1Δg) is produced in massive amounts in the atmosphere, where its quantum yield from O3 Hartley band photodissociation is close to unity.2 This photoabsorption process is the mechanism by which ozone shields the Earth from 200 to 310 nm solar radiationeach O2(a1Δg) molecule generated corresponds to a photon that cannot reach the ground. The O2(a1Δg) state is also an integral component of the high-powered infrared oxygen−iodine laser.3 Nevertheless, relatively little information is available on the kinetic processes involving O2(a1Δg). This lack of data is in part because there is little chance of detecting emission from such levels, since the radiative lifetime of the a1Δg state is 76 min,4 and collisional removal of the vibrationally excited levels is rapid.1 Active probing techniques of O2(a1Δg) have been developed that allow small transient concentrations to be detected. Resonance-enhanced multiphoton ionization (REMPI) has been used to monitor the v = 0 level of O2(a1Δg).5−8 The technique has been extended to vibrationally excited levels, where a critical issue is understanding the spectroscopy of the various multiphoton processes and the nature of the spectral interferences.9−11 Although the study described here involves only the v = 1−3 levels of a1Δg, other investigations at SRI International (SRI) have shown the atmospherically important process of O atom recombination leads to very high O2(a1Δg) vibrational levels, up to at least v = 19.1,12 The rate coefficient values quoted here for a(v = 1, 2) at 300 K were included in an earlier review.1 The actual data and analysis are presented in this report for the first time. More © XXXX American Chemical Society

recent data for a(v = 3) are also included. Moreover, another SRI publication reported measurements of the rate coefficients for removal of a(v = 1) and X(v = 1) by O(3P), O2, and CO2 at 240 and 295 K.13 In that work, a different experimental approach was used, and the kinetics were inferred by a detailed analysis of the rapid collision-induced equilibration of a(v = 1) and X(v = 1) in this system. The v = 1 level of the a1Δg state has been previously studied14,15 using vacuum ultraviolet absorption techniques to monitor the excited O2 molecule. The investigators reached diametrically opposite conclusions as to the mechanism of relaxation of O2 molecules in this level by collisions with ground-state O2. Collins and Husain14 found the v = 1 levels of a1Δg and of the ground-state X3Σ−g are strongly coupled, such that the reaction to produce O 2 (X 3Σ g−, v = 1) and O2(a1Δg, v = 0) and its reverse are very fast. Klais et al.15 concluded from their experiments that O2(a1Δg, v = 1) removal in O2 did not lead to enhanced production of O2(a1Δg, v = 0), a significantly different conclusion than that of the earlier study. We will show the results of Collins and Husain are consistent with the more direct experimental data presented here. In addition to the vacuum ultraviolet absorption measurements described above, Parker16,17 and Parker and Ritke18 reported investigations of O2(a1Δg, v = 1) relaxation in O2 via sound absorption at high pressure. The results are quite confusing, and their conclusion that conversion of O2(a1Δg, Received: August 1, 2018 Revised: September 6, 2018

A

DOI: 10.1021/acs.jpca.8b07469 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A v = 1) to O2(a1Δg, v = 0) is a very inefficient process was later revised to indicate the opposite.16−18 An important question concerns the mode of excitation to generate O2(a1Δg). Having previously studied the low vibrational levels in the b1Σ+g state using direct laser excitation, we note there is much less ambiguity in directly exciting that state from ground-state O2, compared to using O3 photodissociation as a source of O(1D) followed by energy transfer to O2 to generate O2(b, v = 0, 1).19,20 However, the a1Δg levels cannot be generated in such a direct manner because of the small X → a transition probability and the Franck−Condon overlap that favors transitions at equal v, so ozone photodissociation was required. It was ultimately found that the magnitude and the level dependence of the a1Δg (v = 1−3) loss rate coefficients with O2 are quite compatible with expectations based on the model developed for O2 collisional removal of b1Σ+g (v = 1, 2, 3).1,19−21 In this model, the low-v vibrational distribution in the b1Σ−g state is converted to the same distribution in the ground state, that is:

determined. The purities of the gases are 99.995% for O2 and CO2 and 99.999% for N2, and flow rates up to 600 sccm are used. The O3 is produced by a commercial ozonator and trapped on silica gel at 196 K. To deliver the O3 to the cell, a carrier gas, either He or N2, flows through the trap to entrain some of the O3. The gases are supplied into and pumped out of the cell along an axis perpendicular to the axis of the two laser beams and running through the center of the cell. In this manner, the linear velocity of the gas is such that the probed volume is displaced several beam diameters between successive laser pulses. We also performed independent checks to verify that varying the gas flow rate below the measurement conditions does not affect the experimental results. The wavelength thresholds for production of O2(a1Δg, v = 1, 2, 3) from ground-state ozone photodissociation are 296, 284, and 273 nm, respectively, when the other product is O(1 D).1 When undertaking collisional energy transfer measurements, we prefer to work with the highest vibrational level produced in the ozone photodissociation, because in that case cascading from yet higher levels is unimportant. Thus, for measurements on v = 3, 2, and 1, the photodissociation wavelengths are 248/266, 280, and 288/293 nm, respectively. Naturally, all the lower levels are produced as well, and above v = 2 it becomes increasingly difficult to find clean identifiable transitions to exclusively ionize the desired level. Nevertheless, we were successful in locating such a transition for the v = 3 level.

O2′ b1Σ+g , v = 1−3 + O2″ → O2′ X3Σ−g , v = 1−3

(

+ O2 ″

)

b1Σ+g ,

(

(

)

v=0

) (1)

The evidence in support of this energy transfer process stems to a large extent from the measured activation energies and a comparison of E−E and V−V endothermicities. Given the uncertainty and conflicting experimental results and interpretation in the literature, we undertook the present study to clarify some core issues in the relaxation of vibrationally excited oxygen in the a1Δg electronic state. We report laser-based laboratory studies in which O2(a1Δg, v = 1− 3) is generated by O3 photodissociation, and the temporal evolution of these a1Δg levels in gas mixtures containing O2, N2, and CO2 is monitored using REMPI. We determine the relevant collisional deactivation rate coefficients and compare them with available information for the collisional deactivation of the O2(X3Σ+g ) and O2(b1Σ+g ) states. Finally, we discuss the implications of the results for planetary atmospheres.

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RESULTS Ionization Spectra Following Ozone Photodissociation. For REMPI detection, possible background ionizations need to be considered prior to assuming that the ion signal as a function of pump−probe delay represents the temporal evolution of a given state. The gerade Rydberg levels of the d1Πg state through which the 2 + 1 REMPI process takes place have various perturbations and interactions with nearby levels, and extensive work has been done to characterize the spectroscopy of these states.9−11 Figure 1 shows two examples of ionization spectra measured under different cell conditions, where the resonant ionization occurs through vibrationally excited levels of the d1Πg state. The appearance of the two spectra varies with the time delay between the photolysis and ionization laser pulses. Under the conditions used to acquire the upper spectrum of Figure 1, N2 efficiently removes O(1D), while having little effect on vibrationally excited O2 in the a1Δg state, whereas in the lower spectrum, O2 collisions rapidly remove vibrationally excited O2 in the a1Δg state, and the O(1D) efficiently interacts with O2 via the reactions:

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EXPERIMENTAL METHODS The experiments are performed in an octagonal anodized aluminum cell ∼18 cm across, with eight ports around the perimeter for bringing in the laser radiation, for gas handling, and for the electrical connections. O2(a1Δg) is produced by partial photodissociation of O3 in the Hartley band using an unfocused laser pulse with a typical pulse energy of 50−300 μJ and an approximate beam diameter of 1 mm. The timedelayed, pulsed, focused (15-cm focal length lens) output of an excimer-pumped or Nd:YAG-pumped dye laser then ionizes the excited O2 molecules through a 2 + 1 REMPI process, with a typical pulse energy of 4−10 mJ. The photolysis and probe beams are counter-propagated and overlap in the center of the cell. Ions are collected by an electrode biased at −160 V, located ∼5 mm from the focus of the probe beam. The ion signal, presumed to be O2+, is amplified and sent to a boxcar integrator operated with a 15-μs sampling gate width. A programmable delay generator scans the time delay between the photolysis and probe laser pulses, and a computer sums the ion signal at the various delay times. The gases are introduced into the cell through calibrated mass flow meters, and a capacitance manometer gauge measures the total pressure so that accurate partial pressures in the gas mixtures can be

O(1 D) + O2 X3Σ−g , v = 0 → O(3 P) + O2 b1Σ+g , v = 0, 1

(

)

(

) (2)

O2 b1Σ+g ,

(

+

O2 b1Σ+g ,

(

)

v=1 +

O2 X3Σ−g ,

(

)

v=0

)

v=0 →

O2 X3Σ−g ,

(

)

v=1

(3)

Reaction (2) is rapid at room temperature with a rate coefficient of (4.0 ± 0.8) × 10−11 cm3 s−1 and a yield close to unity.2,22 Although the process is only 36 cm−1 endothermic to generate the b(v = 2) level, there is no evidence of its production. An early attempt to determine the relative amounts of the v = 0, 1 levels indicated that 40% of the nascent population was in the v = 1 level,23 but more recent B

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Table 1. Energies (cm−1) of O2 X3Σ−g , a1Δg, b1Σ+g Vibrational Levels Relative to X(v = 0)27 v

X3Σ−g

a1Δg

b1Σ+g

0 1 2 3

0 1556.4 3089.2 4598.7

7889.4 9373.1 10 830.6 12 262.0

13 122.2 14 527.0 15 903.8 17 252.5

The very reactive O(1D) is produced in reaction (4) with high yield and significant translational energy; it interacts very rapidly with many of the possible system components, and there are still questions about the products. However, there is clearly a reactive channel30 O(1 D) + O3 → O2 5 Π g,A , A′, c , a , b , X + O2

(

)

(6)

The rate coefficient for this channel is (1.2 ± 0.4) × 10−10 cm3 s−1, with a parallel reaction producing an oxygen molecule and two oxygen atoms with a similar rate coefficient.2 In the present experiments, many of these species are observed in an ozone/rare-gas system, and vibrationally excited O2 is also a known product.31 Although the reaction products may not be kinetically important, adding extra components increases the chance the ionization probe laser will detect them, creating ambiguities. As mentioned, the technique becomes more complicated as the vibrational level increases. For the v = 3 level, the ionization laser had to be shifted to shorter wavelengths, because using the d-a 4−3 transition was more favorable than the 3−3 band. Figure 2 shows the 4−3 band, which is significantly weaker than the 3−3 band. The relatively low signals obtained in the experiments resulted in slower data accumulation.

Figure 1. Ion signals as a function of laser wavelength following photodissociation of ozone at 266 nm. The top spectrum was obtained at 9.6 Torr of N2 and ∼100 mTorr of O3 with a 20 ns delay between the photodissociation and ionization laser pulses. The 2 + 1 REMPI features are assigned to the 2−2 and 1−1 vibrational bands of the d1Πg−a1Δg electronic transition. The bottom spectrum was obtained at 12.6 Torr of O2 and ∼150 mTorr of O3 with a 150 ns delay between the two laser pulses. The 2 + 1 REMPI features are assigned to the 4−1 and 3−0 bands of the d1Πg−b1Σ+g electronic transition. The wavelength scale is approximate; refer to Morrill et al. for exact photon energies of the ionization features.11

measurements found a value close to 80%,24 which is consistent with analysis of space-based measurements.25 Reaction (3) is also rapid and reversible and has a rate coefficient with a value of (1.52 ± 0.04) × 10−11 cm3 s−1 in the forward direction.26 Figure 1 also illustrates that the environment in which these experiments are performed is complex and that photodissociation of ozone leads to a variety of active species. Unless a way is found to remove them, the experiments are difficult to interpret because of the overlapping transitions and possible complications from secondary reactions. In the primary photodissociation step, there are two pathways:2 O3 + hv → O2 (a1Δg , v = 0−m) + O(1 D)

ϕ ≈ 0.9 (4)

O3 + hv →

O2 X3Σ−g ,

(

)

3

v = 0−n + O( P)

ϕ ≈ 0.1

(5)

Figure 2. Scan of the ionization wavelength region in which a(v = 0−3) are sampled. The weak signal from the a(v = 3) level is probed at 325.287 nm on the d-a 4−3 band, and the off-resonance signal is obtained at 325.624 nm.

where m and n are the maximum vibrational levels that are energetically possible given the energy of the photodissociation photon, and ϕ is the fraction of the total reaction that proceeds via that channel. Table 1 shows the energies for the lowest vibrational levels of the first three electronic states of O2. The relative yield of processes (4) and (5) is independent of wavelength between 230 and 300 nm.2 Although the yield from reaction (5) is small, the product O2 is excited to a wide range of vibrational levels, all the way to the thermodynamic limit, cf., to v = 26 for dissociation at 248 nm.28,29 Thus, the number of vibrational quanta in the system can be quite large.

Additional points of interest that also represent potential complications are presented in Figure 3, which displays a REMPI spectrum acquired following photodissociation of ozone in an argon buffer at 293 nm. At this wavelength, O2(a1Δg) can initially be produced only up to v = 1. The O2 signal in the d-a 1−1 band at 340 nm is strong when the delay time is 100 ns (upper spectrum), but other features are also C

DOI: 10.1021/acs.jpca.8b07469 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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directions are very fast in reality. N2 does not break the link between a(v = 1) and X(v = 1), since neither is significantly removed by N2 collisions. In O3/O2/N2 mixtures, a(v = 1) behaves in a complex manner and persists in the system long after the measured rate coefficient for a(v = 1) + X(v = 0) indicates it should be gone. By performing the O2 measurements in CO2 buffer, this problem is alleviated. Long-term transients are not observed in that case, and we conclude the effect of CO2 is not only to quench O(1D) rapidly but also to remove X(v = 1). The rate coefficient for this process at 300 K is on the order of 1 × 10−14 cm3 s−1.13,33 In the recent study by Pejaković et al.,13 N2 is used as a buffer, and a complete kinetic treatment is considered. The results are in agreement with the rate coefficients for a(v = 1) presented here. For the quenching rate coefficient of a(v = 1) by N2, Collins and Husain14 found a value of 3 × 10−16 cm3 s−1 as an upper limit, and a similar upper limit for X(v = 1) was measured simultaneously. These results have been confirmed by Pejaković et al.13 Vibrational equilibration is not unique to molecules in the v = 1 vibrational levels; similar equilibration will occur for the v = 2, 3 levels with increased complexity. The relaxation of a(v = 2) can proceed via two near-resonant energy transfer pathways:

Figure 3. Ion signals as a function of laser wavelength following photodissociation of ozone at 293 nm. The top spectrum was obtained at a 100 ns delay between the photodissociation and ionization laser pulses. The strongest REMPI feature (between 339 and 340 nm) is assigned to the 1−1 vibrational band of the d1Πg− a1Δg electronic transition. The vibrational band labels above the top spectrum correspond to vibrational bands of the two-photon 5Πg−5Πg transition. The 8−0 band is the strongest and overlaps with the 1−1 band of the d1Πg−a1Δg electronic transition. The bottom spectrum was obtained at a 46 μs delay between the two laser pulses. The bulk of the REMPI features are assigned to ionization via the d1Πg−a1Δg electronic transition. The 0−0 band is weak, since it is a 2 + 2 REMPI feature, whereas the other bands are 2 + 1 features. The wavelength scale is approximate; refer to Morrill et al. for exact photon energies of the ionization features.11 Several features in both spectra remain unassigned.

O2 (a1Δg , v = 2) + O2 X3Σ−g , v = 0 → O2 X3Σ−g , v = 2

(

(

+ O2 (a Δg , v = 0)

)

(8)

O2 (a1Δg , v = 2) + O2 X3Σ−g , v = 0 → O2 X3Σ−g , v = 1

(

1

+ O2 (a Δg , v = 1)

)

(

)

(9)

where reactions (8) and (9) are 147 and 99 cm−1 endothermic, respectively. By energetics alone, it is unclear which energy transfer process will dominate the removal of O2(a1Δg, v = 2). Both processes can quickly establish equilibrium between the states involved in the energy transfer. O2(a1Δg, v = 1−3) + O2 Rate Coefficient Measurements. The method for obtaining the temporal evolution of O2(a1Δg, v) is illustrated in Figure 4 with sample data probing O2(a1Δg, v = 1) in a background of CO2. The ion signal is measured as a function of delay time between the two laser pulses. The top curve in Figure 4 shows the ion signal when the ionization laser is on-resonance at 339.70 nm (air) with the P-branch bandhead of the d-a 1−1 band. For the off-resonance delay scan, the ionization wavelength is detuned 0.64 nm to the blue. When the ionization laser fires before the ozone photodissociation laser pulse, a small ionization signal is obtained at negative delay times, as can be seen in Figure 4. A dashed line is drawn at that ion signal level. Figure 4 also shows the signal obtained off-resonance with the d-a 1−1 feature. This signal is subtracted from the onresonance signal, and the difference is shown in the bottom panel. The off-resonance signal shows no significant deviation from the dashed baseline at delay times greater than 1 μs. The difference signal in the bottom panel is assigned to the temporal evolution of O2(a1Δg, v = 1). From this figure alone, the identity of the a(v = 1) quencher is not obvious, but it is subsequently shown to be O2. The CO2 pressure was subsequently standardized at 60 Torr to perform all the O2 collider measurements. Fortunately, the O2 rate coefficients are much faster than those for CO2, so working at a relatively high CO2 pressure did not cause the

present. In particular, the v″ = 0 progression of the O2(5Πg) state, which can only originate with reaction (6), can be seen.1,32 The 6−0, 7−0, and 9−0 bands of this progression are tentatively identified, with the 8−0 band lying on the blue edge of the d-a 1−1 band. Such molecules rapidly appear and then disappear, and after a 46 μs delay (lower spectrum), the quintet O2 molecules are gone, with the remaining structure in the 336.5−338 nm region assigned to the d-a 2−2 band (Figure 1). The fact that a(v = 2) is detected is an indication of subsequent energy transfer. The vibrational manifold of the quintet state levels has been previously identified from experiments in which collisional transfer occurred from the O2 Herzberg states,1 but this is the first demonstration that the quintet state is produced via reaction (6). To avoid the complexities stemming from this reaction, we must have an O(1D) quencher in the system. We investigated two O(1D) quenchers in this study, N2 and CO2. CO 2 removes O( 1 D) with a rate coefficient of (1.1 ± 0.2) × 10−10 cm3 s−1, ∼3.5 times faster than N2.2 In initial experiments, N2 was used to ensure that O(1D) was removed, but an additional issue is the rapid equilibration a(v = 1) + X(v = 0) ↔ a(v = 0) + X(v = 1)

)

1

(7)

Unless the vibrational quantum is removed from the system, the forward reaction will appear to be slow, while both D

DOI: 10.1021/acs.jpca.8b07469 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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Figure 5. Ion signal from O2(a1Δg, v = 2) as a function of delay time for several CO2 partial pressures. All data were taken at He partial pressures near 25 Torr and O3 partial pressures near 150 mTorr prior to dissociation. (□) The experimental data for 22, 110, and 200 Torr of added CO2, respectively. The solid lines show the best singleexponential fits to the data allowing the baseline to float. The dotted lines are the baseline determined by averaging the experimental data when the probe laser pulse precedes the photodissociation laser pulse.

Figure 4. Ion signal as a function of delay time following photodissociation of ∼100 mTorr of ozone in a mixture of 33 Torr He, 60 Torr CO2, and 2.3 Torr O2. (□, top) The ionization laser is tuned to the d-a 1−1 band. (○, center) The laser is detuned offresonance. The amplitude of the data indicates the relative magnitude of the ion signals. (△, bottom) The difference between the onresonance and off-resonance signals and corresponds to the temporal evolution of O2(a1Δg, v = 1). The dashed line shows the baseline for each measurement.

hope for larger signals at v = 3. This was inconclusive, because the ionization signal from ozone itself swamped the O2 signal. The v = 3 data sets were instead obtained using 266 nm photodissociation, and although longer integration times were needed, the data quality was quite satisfactory. Figure 6 shows plots of the decay rate or decay constant as a function of O2 partial pressure, the slopes of which give the

a(v) molecules to disappear inordinately rapidly. Figure 5 presents the temporal evolution of O2 molecules in the a1Δg(v = 2) level for different O2 partial pressures. For the v = 3 measurements, ozone was photodissociated at 248 and 266 nm to create the O2(a, v) distribution, while the second laser excites a(v = 3) molecules via two photons to the v = 4 level of the O2(d) state, from which ionization occurs by absorption of a third photon. Because of small but nonnegligible deviations from single-exponential behavior, the data were analyzed in different ways to reflect possible choices for how the baseline was treated. The values of the rate coefficient obtained vary somewhat depending on the chosen approach: (1.63 ± 0.09) × 10−11 cm3/s for fits to a single-exponential function with the baseline forced at zero, (1.89 ± 0.14) × 10−11 cm3/s for fits to a single-exponential function with a floating baseline, and (2.14 ± 0.16) × 10−11 cm3/s for fits using a double-exponential function. The rate coefficient for the process O2 (a , v = 3) + O2 → O2 (X , v = 3) + O2 (a , v = 0) (10)

appears to be consistent with the data obtained for the a(v = 1, 2) levels.1,13 Earlier work on the O2(b, v) levels and O2 collider gas shows that, for v = 1−3, the rate coefficients decrease with increasing v in a similar manner to the picture that has emerged for O2(a, v = 1−3).19−21 As mentioned earlier, the data for a(v = 3) was somewhat more difficult to obtain because of the smaller ionization signal. An attempt was made to use photodissociation at 193 nm in

Figure 6. Decay constants for O2(a1Δg, v = 1−3) as a function of O2 partial pressure. E

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The Journal of Physical Chemistry A Table 2. Rate Coefficients for Removal of O2(a1Δg) as a Function of Vibrational Levela rate coefficients, cm3 s−1 vibrational level, v

O2

0b

(1.7 ± 0.1) × 10−18 ≥1.7 × 10−11 c (5.6 ± 0.6) × 10−11 d (5.6 ± 1.1) × 10−11

1

2

(3.6 ± 0.4) × 10−11

3

(1.9 ± 0.4) × 10 −11

CO2

N2

He

≤3 × 10−20

≤1 × 10−20

≤1 × 10−20

(1.5 ± 0.2) × 10−14 d (1.9 ± 0.2) × 10−14 f (1.8 ± 0.4) × 10−14 (2.4 ± 0.2) × 10−14 f (4.4 ± 0.6) × 10−14 (2.7 ± 0.3) × 10−14 f

0).

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3

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. (T.G.S.) *E-mail: [email protected]. (K.S.K.) ORCID

Konstantinos S. Kalogerakis: 0000-0002-3937-8305 Present Address ‡

Air Force Research Laboratory, Kirtland AFB, New Mexico, United States Notes

The authors declare no competing financial interest. † Previously affiliated with SRI International.

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ACKNOWLEDGMENTS We acknowledge significant contributions by R. A. Copeland and D. A. Pejaković in the development of the relevant experimental techniques, data analysis, and interpretation of the results. These studies have been supported by various NASA and NSF programs over a period of years, most recently by NSF Award Nos. AGS-1651450 and AGS-1441896.

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REFERENCES

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DOI: 10.1021/acs.jpca.8b07469 J. Phys. Chem. A XXXX, XXX, XXX−XXX